The usual way to define a type of an object in TypeScript is using an object type:
or an index signature:
These are good ways to define object types.
But Record<K, V>, the third approach, has the benefit of being shorter and more readable. Let's see how to use it in your code.
1. Record type
Record<K, V> is a generic type that represents an object type which keys are K and values are V.
For example, Record<string, number> is an object type with string keys and number values:
Record<string, number> is permissive regarding the object structure, as long as the keys are strings and values are numbers:
But Record<string, number> throws a type error if the value of a prop is a string:
There are 2 simple rules to remember regarding the allowed types of the keys and values. In Record<K, V>:
- but there is no restriction on the value type
V
Let's see some valid record types:
Types like boolean, object, Function, etc. are not accepted as keys:
2. Record with union key
As seen above, Record<string, number> permits any key names in the object. But quite often you need to annotate objects with a fixed set of keys.
The record type accepts a union type as a key, which is useful to fixate the keys.
A union of string literals is a common way to define the key type:
For example, Record<'annual' | 'bonus', number> represents an object which can have only annual and bonus keys:
Using less than necessary or keys than aren't in the union is prohibited:
The record type with union keys is equivalent to the regular object type. The record type has the benefit of not repeating the value type (like the regular object does):
3. Record benefits
I prefer record type instead of index signature most of the time. Record syntax is shorter and more readable (altought it's also a matter of taste).
For example, the record parameter is a bit easier to grasp than the index signature parameter:
Compared to record type, the index signature doesn't accept literals or a union as key type:
4. Conclusion
Record<K, V> is an object type with key type K and value type V.
The key type K can be only number, string, or symbol, including their literals. On the value type V is no restriction.
To limit the keys to a specific set, you can use a union of string literals Record<'key1' | 'key2', V> as the key type.
Check also my post index signatures to learning more about object types.
How often do you use record type?